In chemical engineering, biochemical engineering and protein purification, cross-flow filtration (also known as tangential flow filtration) is a type of filtration where the majority of the feed flow travels tangentially across the surface of the filter, rather than through the filter. The permeate passes through the filter membrane or filter media, while the retentate exits the filter body carrying the particles too large to pass through the filter element. A range of different types of membranes are available that can be characterized by the size of the particles and solutes that do not pass through the membrane. For example, in biological applications, plasma filters have microfiltration membranes that retain cells and other particles larger than about 1 μm in the retentate, while allowing smaller particles (e.g., proteins) to pass through with the permeate. Also, ultrafilters—filters used to concentrate proteins—retain particles larger than 10 kiloDalton. Nanofiltration generally involves filtering with a pore size of about 0.001 micron, and can remove most organic molecules, viruses, salts and divalent ions. Reverse osmosis—the most restrictive filtration with filters having a pore size of about 0.0001 micron retains all particles and solutes including monovalent ions and most minerals. The principal advantage of cross-flow filtration is that the filtered particles and solute are substantially washed away from the filter surface during the filtration process, increasing the length of time that a filter unit can be operational.
Flow systems typically control certain conditions such as, for example, pressure and flow rate, to achieve a preferred performance. The blood pumping system of an extracorporeal blood purification system, for example, is selected or configured so that the range of pressure and flow rate may be preferentially optimized for the characteristics of a given patient access. However such systems may have particular components (such as a plasma filter in the case of an extracorporeal blood treatment system) whose optimal operating conditions differ from the preferred operating conditions of the system as a whole. In the following description, the term solute is intended to broadly include molecules, compounds or other substances dissolved in the fluid. Examples of solutes include but are not limited to salts or sugars. In the following description, the term particle is intended to broadly include compounds, polymers, solid structures, biological cells, parts of cells, macro molecules, protein structures, etc. that are carried by the fluid in a mixture or in suspension. Examples of particles include but are not limited to whole blood cells, bacteria, viruses and proteins.